The present invention relates to an improved strapping joint and the method for providing the improved joint. More specifically the invention provides an improved joint structure in sealless joints of thermoplastic strapping.
In the recent past, Signode Corporation, the assignee of the entire interest in the present application, has developed several processes, tools and power strapping machines for joining the overlapping end portions of a tensioned thermoplastic strap loop by friction-fusion techniques, and these methods and apparatuses are typified by those disclosed and claimed in U.S. Pat. Nos. 3,331,312, 3,442,203, 3,442,732, 3,442,733, 3,442,734, 3,442,735, 3,494,280, 3,586,572, 3669,799, 3,679,519 and 3,718,526.
The thermoplastic strap material used in the practice of the inventions disclosed in the above named patents have conventionally been oriented nylon, polypropylene and polyester. Straps formed from such materials are fabricated by generally similar techniques, as is disclosed, for example, in U.S. Pat. No. 3,394,045. According to this patent, polypropylene sheet material, which is subsequently sliced into straps, is stretched to obtain planar orientation of the micromolecular chains. The molecular orientation of the polypropylene resin in the sheet substantially increases the tensile strength along the lines of orientation. However, this also renders the opposing surfaces of the sheet susceptible to abrasion which could ultimately interfere with its use in a strapping apparatus. To overcome this, the surfaces of the sheet are subjected to flash-heating to effect fusion of the outer opposing layers but insufficient to cause fusion in the entire thickness of the sheet. The flash heating acts to smooth the opposing surface layers, but does not disrupt the molecular orientation of the rest of the sheet material so that the high tensile strength is retained. The formed sheet material is then sliced to produce straps. Slicing is done generally parallel to the molecular lines of orientation so that the straps have high tensile strength along their length. This makes excellent straps for use in the packaging of materials where a strap is drawn tightly around a package with overlapping end portions secured to each other in the form of a ligature. The overlapping end portions can be secured by securing means, which include strap seal, adhesive or weldment such as friction fusing or thermoweld.
One of the characteristics of fused joints formed in accordance with the teachings of the above patents is their exceptional tensile strength (i.e., resistance to forces applied in the direction of the length of the strap). A further characteristic of such joints is that the peel strength (i.e., resistance to forces applied normally to the length of the strap) is relatively low. Although the low peel strength of friction-fused joints is very useful in enabling the strap to be removed from the strapped article, on occasion it may also result in unintentional opening of the joint. For example, if the end of the outermost strap portion is snagged in the handling of the article the joint could peel open. It has also been found that peel forces are more apparent for thicker strap as opposed to thinner strap material. However, it is also an ongoing problem that some weld joints are subject to premature failure.